Around 300 British citizens who joined Isis in Iraq and Syria are thought to have evaded capture and fled into Turkey, according to a new report. With the group having lost vast swathes of ground, including the central cities of Raqqa and Mosul, many fighters have simply fled the front line - with thousands heading to Turkey.

Turkey was known as a route through which people wishing to join the Islamic State were likely to travel. According to The Times, though many Isis fighters have been captured or killed, many more have escaped. According to The Times, of the around 850 British people who joined Isis about half returned while nearly 130 are confirmed to have died.

One Syrian-Kurdish intelligence officer told the paper that though they had jailed a number of French fighters, "we think most of the British have escaped".

Concerns have been raised that fighters returning from the front line could carry out terror attacks in the name of Isis once they return to Europe.

Counter-terrorism expert David Otto told Newsweek that "Jihadists don't just leave Jihad behind just because their Caliphate has collapsed. Displaced fighters can't just disappear, they have to seek for an alternative platform. Fleeing jihadists go where they can easily blend, hence Britons will go to Turkey, Africans to Libya, and so on."

A new Islamic State propaganda video said to originate from Somalia and released on Christmas Day called for lone-wolf attacks on Westerners celebrating during the Christmas season and on New Year's Eve.

In a video including images of cars ramming into revellers and a train derailing, an Isis fighter calls for jihadists to "hunt down" non-believers during the holiday season.

The UK Foreign Office told those travelling in Europe over the break to stay "vigilant", adding there would likely be increased security in places with large crowds and holidaymakers should "follow the advice of local authorities".

In London, the Mayor's Office similarly urged vigilance, adding that ticket holders for the annual fireworks display would have photo IDs and bags checked. A quarter of a million people are estimated to be attending the New Year's festivities on the banks of the river Thames, with tickets selling out.